The present invention pertains to actuators and, more particularly, to an actuator especially suited to drive a member such as a paper machine headbox slice opening control spindle.
As illustrated in FIG. 1, a typical paper machine headbox 1 distributes pulp slurry or stock through a long horizontal slit opening 2 onto a traveling perforated web or "wire" 3. Transverse the direction of wire travel, paper density or "weight" can be changed by opening or closing a long stainless steel bar or "slice lip" 4 which comprises the top of the slit opening and, therefore, determines its height. Attached to the slice lip 4 are spindles or "slice rods" 5 which are typically spaced about 3 to 6 inches apart. By exerting a linear force through these spindles, "weight actuators" 6 can elastically deform the slice lip 4 into a shape which produces a slit opening that yields a paper sheet having a preselected weight that is uniform across the sheet.
Historically, the weight actuators driving the spindles were manually controlled. In recent years, however, automatic weight actuators have become available which operate in response to signals generated by weight, moisture and thickness sensors to precisely control distribution of pulp stock onto the wire.
To be effective, an automatic weight actuator must deliver a force ranging from 500 to 4,000 pounds. This force must be delivered in quick, precise, very small steps. At the same time, the actuator must be very small so that it can fit into and onto a multitude of headbox styles which have many and varying nearby encumbrances. Also, because downtime on a paper machine is very expensive, it must be very easy to install and service.
Many existing weight actuators utilize a pneumatic or electric motor driving through a gear reduction to a keyed power screw which, in turn, is mechanically connected to the slice rod.
Due to the normal machining tolerances of the headbox, actuator and slice lip elements, the slice rod often does not line up exactly with the centerline of the actuator drive train. Such misaligned connection imparts a side load on the actuator which can impair and reduce its force producing capability.
Misalignment can also result from a non-level mounting surface for the actuator, incorrectly located actuator mounting holes, a connector that does not attach the actuator and slice concentrically and in proper alignment, or a slice lip connection that does not properly align the slice rod with the actuator. These conditions can require that considerable installation time be devoted to shimming and adjusting the actuator mounting location to produce the best possible alignment.
Existing actuators utilize linear bearings on the output shaft to minimize the transfer of side loads to the actuator mechanism. However, the short length and travel of the actuators requires that these bearings be short and closely spaced, whereby their effectiveness is limited.
Existing actuators utilize a ball screw or power screw to the convert rotary motion of the motor into linear output motion. Such actuators require some type of torque restraint on the output screw or nut to prevent it from turning. Often, this is accomplished by a key, on the output screw or nut riding in a longitudinal slot formed in the actuator housing. To minimize friction and avoid binding, both the key and the slot must have precisely machined surfaces that are properly aligned. This adds complexity and, hence, cost to the actuator, as well as increasing its size.
Below the actuator output shaft, a mechanical connection must be provided between the shaft and the slice rod. Ideally, this connection is made with the actuator at the midpoint of its travel range and the slice lip level. As previously noted, various factors combine to render the precise geometry of each installation unique. To allow for these variations, a turnbuckle or smooth-bore clamp is commonly employed as the connector. However, both of these types of connectors must be precisely machined and aligned to minimize side loads and both increase the cost and size of the actuator.
During installation of weight actuators, the orientation and position of the actuator itself is determined by the hole pattern of its mounting plate, as are the positions of connectors for associated electric power cables or compressed air lines. When installing actuators to some headboxes, special adaptor plates are required to position the actuator so as to clear local obstructions. Thus, substantial planning and custom design may be required to effect some installations.
Finally, to achieve the necessary precision, it is desirable to minimize or, if possible, eliminate any power screw backlash. Feedback control systems that measure actuator output position electronically compensate for small amounts of internal backlash but, nevertheless, existing actuators commonly utilize antibacklash components to compensate for the lack of precision machining. These components may comprise two ball nuts or power screws (instead of one) which are adjusted away from each other until the backlash is taken up and then either locked in place or preloaded by a spring. This also adds complexity and cost to the actuator, as well as increasing its length.
It is, therefore, a primary object of the present invention to provide a linear weight actuator which overcomes the aforesaid limitations of the prior art.
This is accomplished, in general, by an actuator wherein a clamp assembly adjustably connects the power screw to the output spindle. The assembly includes an integrally formed key, riding in a slot formed in the actuator housing, which obviates the need for a separate key element.
The actuator further includes a mounting plate adjustably clamped to the actuator housing, by a threaded mounting nut, such that the plate may be conveniently rotated relative to the housing. A linear bearing disposed in the mounting nut provides lateral support for the output spindle at a point spaced from the clamp assembly.
The actuator still further includes a clamping ring encircling a thinned segment of the power nut and urging it against the power screw to prevent backlash.